Error if missing submodules

[Ryanf55]

Purpose

Make it clearer if you forget the submodules

Before

CMake Error at CMakeLists.txt:25 (include):
  include could not find requested file:

    CompilerSettings

CMake Error at CMakeLists.txt:26 (include):
  include could not find requested file:

    OMPLUtils

CMake Warning (dev) at CMakeLists.txt:53 (find_package):
  Policy CMP0167 is not set: The FindBoost module is removed.  Run "cmake
  --help-policy CMP0167" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Error at CMakeLists.txt:83 (find_boost_python):
  Unknown CMake command "find_boost_python".

-- Configuring incomplete, errors occurred!

After

CMake Error at CMakeLists.txt:20 (message):
  Missing /home/ryan/Dev/ompl_ws/omplapp/ompl/CMakeModules.  Did you check
  out the submodules?

[mamoll]

Looks good to me. I added a suggestion for those less familiar with git.

[Ryanf55]

Looks good to me. I added a suggestion for those less familiar with git.

Thanks for the review. Good idea!